Question

A compound contains total of three optical isomers. The correct statement for these optical isomers is (A) All are active (B) One isomer active and two isomers inactive (C) Two isomers active and one isomer inactive (D) All of the above

Short answer

The correct statement for these optical isomers is (B) One isomer active and two isomers inactive. This can happen if the compound is chiral, with one enantiomer and the racemic mixture considered as the second and third isomers.

Step-by-step solution

Understand Optical Isomerism and Optical Activity

Optical isomerism is a form of isomerism, or the occurrence of different compounds with the same molecular formula, which occurs due to differences in the orientation of molecules in space. Optical isomers can rotate plane-polarized light, which allows us to differentiate between different isomers. If a compound rotates plane-polarized light, it is said to be optically active.

Consider the Options for the Isomers

Given that there are total three optical isomers for this compound, it means either the compound itself is chiral with two enantiomers and their racemic mixture, or the compound has two chiral centers, providing three stereoisomers. A racemic mixture is optically inactive because the rotations of the two enantiomers cancel each other out.

Evaluate the Provided Options

(A) If all isomers are active, this would imply that all three are chiral and different from each other. Since there are only two enantiomers for one chiral compound, this option is not feasible. (B) One isomer active and two isomers inactive could happen if the compound is chiral, with one enantiomer and the racemic mixture considered as the second and third isomers. (C) Two isomers active and one isomer inactive is not possible, because if two isomers are active, they must be enantiomers of each other and their mixture must be the third isomer, which should be inactive. (D) "All of the above" cannot be correct because options (A) and (C) aren't feasible. Therefore, answer (B) One isomer active and two isomers inactive is the correct statement for these optical isomers.

Key concepts

Optical Activity

The phenomenon of optical activity is truly fascinating. It refers to a substance's ability to rotate the plane of polarized light. Polarized light moves in a single direction, but when it passes through an optically active substance, it exits with a changed direction. This rotation can be either to the left, known as levorotatory, or to the right, called dextrorotatory.

The degree of this rotation is measured with a tool called a polarimeter, and it's unique for each optically active compound. This property is fundamental in differentiating between compounds that have the same molecular formula but behave differently with light — a concept known as isomerism. This leads us to understanding chirality, an intrinsic property that imparts optical activity to a molecule.

Chirality

Chirality is a geometric property of some molecules, indicating they cannot be superimposed on their mirror image. Imagine your hands; they are mirror images of each other but cannot be perfectly aligned if superimposed — like left and right gloves. In chemistry, if a molecule has a carbon atom with four different substituents, it's often chiral and exists as two mirror images or enantiomers.

These chiral molecules are not symmetrical, and it's this lack of symmetry that allows them to interact with polarized light, leading to optical activity. Just as your hands are functionally similar but not identical, enantiomers can have significant differences in how they react with other chiral substances, such as biological molecules in your body.

Enantiomers

Enantiomers are a pair of molecules that are mirror images of each other and non-superimposable - much like your left and right hands. Although they share the same molecular formula and connections between atoms, their spatial arrangement differs. This 'handedness' in molecules results in different behaviors when interacting with polarized light, where one enantiomer will rotate light in one direction (left or right), and the other will rotate light equally but in the opposite direction.

These slight, mirrored differences have significant consequences in many fields, including pharmaceuticals, where one enantiomer may be therapeutic while the other could be ineffective or even harmful. Understanding and identifying enantiomers is crucial in the development and application of many chemical products, emphasizing the importance of optical activity in scientific studies.

Racemic Mixture

A racemic mixture is a 50/50 blend of two enantiomers of a chiral molecule. Think of it as a mixture of equal amounts of left and right gloves. Despite containing two optically active compounds, a racemic mixture does not exhibit optical activity because the effects of the two enantiomers cancel each other out. This is known as external compensation.

In terms of optical isomerism, a racemic mixture is interesting because it contains optically active isomers but is itself optically inactive. This peculiar aspect of racemic mixtures is crucial, especially in the pharmaceutical industry, where the isolation of a single enantiomer from a racemic mixture can mean the difference between a useful drug and a harmful one.